Support 'pan' feature

[dholroyd]

If AD_descriptor's pan_byte is non-zero, have adcontrol do something useful (and give it ad_sink+ad_src pads to allow it to manipulate the audio).

The audiopanorama element could probably be embedded to implement the behaviour (but in manual testing, method=psychoacoustic made centred output without any panning come out too quiet, so probably method=simple is the way).